This invention relates to a metallic sheath used for prestressed concrete structures, and more specifically, to a metallic sheath for a posttensioning method, which is provided with a means for decreasing frictional resistance between a tendon and a metallic sheath when the tendon is subjected to a tensioning operation.
Generally, when a prestressed concrete structure is executed by a posttensioning method, metallic sheathes are normally used because a tendon is arranged. The metallic sheath has a function as a cover for the tendon so that the tendon is insulated from concrete. After the concrete is cured the tendon is tensioned. The metallic sheath allow the tendon to be arranged smoothly, and have strength enough to withstand a collapse thereof or formation of holes therein when the concrete is placed. Frictional resistance resistance must be small when the arranged tendon is prestressed.
In particular, the frictional resistance between the tendon and the metallic sheath, when the tendon is prestressed, ought to be zero unless both the metallic sheath and tendon are arranged in straight and come into contact each other. However, if it is difficult to arrange the tendon and metallic sheath, and in actual practice they have a slight wave or bend, thus producing an unavoidable frictional resistance therebetween. Moreover, when the metallic sheath and tendon are arranged in a curved fashion, a frictional resistance proportional to the bend-up angle is applied thereto, resulting in a greater frictional resistance therebetween. Furthermore, if the internal peripheral surfaces of the metallic sheath and/or tendon are rusted, a greater frictional resistance therebetween results.
Since the frictional resistance produced when the tendon is prestressed appears as a frictional loss in the prestressing force introduced into the tendon, when a predetermined prestress (which is a prestress contemplated in design) is introduced into a concrete structure, the frictional resistance influences thereon as a decrease in said introduced prestressing force. That is, if the frictional resistance is small, a difference between the prestressing force at the end of the tendon and the prestressing force introduced into the tendon decreases, whereby a predetermined prestress may be provided. Conversely, if the frictional force is great, the difference between the prestressing force at the end of the tendon and the prestressing force introduced into the tendon increases, whereby the prestressing force at the end of the tendon must be made greater to provide a predetermined prestress.
Decreasing the frictional resistance is very important as the following effects are caused: (1) the prestressing force effective to the tendon may be introduced over the full length thereof, (2) the number of tendons used may be reduced, (3) the diameter of the tendons used may be reduced, and (4) the limit in length over which the prestressing force is effective to the tendon may be extended.
In view of the foregoing, means for reducing the frictional resistance (for example, water-soluble oil or soapy water is poured into the sheath, and the like) have been heretofore proposed. However, the previously used methods suffer from various disadvantages. The effect of lowering friction is small (in case of water-soluble oil, the coefficient of friction is 0.29), and is immediately lost. Moreover, the characteristics of the metallic sheath itself (flexibility, hardness or the like) are impaired, and the work required is time consuming.